1. Field of the Invention
The present invention relates to the field of capacitors and printed circuit boards. In particular, it relates to polymer-ceramic composite materials for use in the formation of capacitors and printed circuit boards. The inventive composite materials exhibit a low change in temperature coefficient of capacitance (TCC) in response to temperature changes, as well as other desirable properties.
2. Description of the Related Art
As the circuitry design of central processing units (CPU) seeks to achieve increased operational speed, the performance of integrated circuits becomes ever more important. The circuitry design of printed circuit boards, which mount these integrated circuits, is also very important.
Capacitors are common elements of printed circuit boards and other microelectronic devices. They are used to steady the operational power supply of such devices. Capacitance is a measure of the energy storage ability of a capacitor. A capacitor introduces capacitance into a circuit and functions primarily to store electrical energy, block the flow of direct current, or permit the flow of alternating current. Typically, capacitors comprise a dielectric material sandwiched between two electrically conductive metal layers, such as copper foils. In general, the dielectric material is coupled to the electrically conductive metal layers via an adhesive layer, by lamination, or by vapor deposition.
Heretofore, capacitors arranged on the surface of printed circuit boards have been common. In recent efforts to miniaturize capacitors, it has been known to either use a dielectric ceramic material with a high dielectric constant, or to decrease the thickness of dielectric ceramic layers. The capacitance depends primarily on the shape and size of the capacitor layers and the dielectric constant of the insulating material. In one known arrangement, “embedded” capacitors comprising thin, double-sided copper clad laminates have been formed within multilayered circuit board layers, producing excellent characteristics. Printed circuit boards having such embedded capacitors are able to maximize the surface area of the circuit board for other purposes, and achieve increased signal transmission speed.
Capacitors with high capacitance density are particularly desirable. The capacitance density of dielectric materials can be increased by the addition of ceramic materials. However, the high loading of ceramic filler materials into a dielectric material often results in a composite which is brittle, and which has very low mechanical and processing properties. Such high capacitance density materials are also known to exhibit large changes in capacitance due to changes in temperature. Additionally, materials having high dielectric constants are also known to be sensitive to temperature changes. Materials with such temperature dependencies of capacitance are known to have a high “temperature coefficient of capacitance” or TCC. A material's TCC indicates its maximum change in capacitance over a specified temperature range. Conventional dielectric composite capacitor materials have been developed which have a change in TCC of from as low as ±15% to as low as ±10% over a temperature range of from about −55° C. to about 125° C. However, a need exists in the field of printed circuit boards to develop capacitor materials, and particularly embedded capacitor materials having a very low change in TCC in the range of from about −5% to about +5%, and preferably as low as from about −0.5% to about +0.5%, in response to changes in temperature within the range of from about −55° C. to about 125° C. The present invention provides a unique polymer-ceramic composite material which achieves this goal. The inventive composite materials additionally have excellent mechanical properties such as good peel strength and lack of brittleness, electrical properties such as high dielectric constant, and processing characteristics such as ease of mixing.